The invention relates to a rotating nozzle structure of an extruder which comprises a rotatable nozzle, a die, which rests against the extruder frame and comprises an annular front surface on the nozzle side, and a sealing plate, which is arranged between the front surface of the die and the nozzle and pressed tightly against the sealing surface of the nozzle to seal the nozzle with respect to the die.
The rotating nozzle structure described above can be used e.g. for extruding a groove element used as the core element in optical cables. When such a groove element is extruded, part of the nozzle structure has to be rotatable so that the grooves of the groove element can be provided with the desired helical twist or reverse twist. Such helical or reverse twist is necessary to avoid subjecting optical fibres, fibre ribbons or the like to excessive tension in a finished cable when the cable is bent.
In such a rotating nozzle structure problems are caused by the sealing between the fixed part and the rotating part of the nozzle structure. On the one hand, sealing pressure acts on the sealing surface between the rotating part and the fixed part and it has to be sufficient to provide sealing. On the other hand, this joint is rotatable, in which case the sealing conditions on the sealing surface vary considerably depending on the speed of rotation. To be more specific, the pv value (surface pressure x sliding speed) of the sealing surface has to be kept within acceptable limits with respect to the useful life of the sealing surface, but it is also necessary to provide sufficient sealing strength. At high temperatures in particular, e.g. when the above-mentioned groove element is extruded, it is very difficult to provide a sufficiently durable and tight rotating sealing surface. No matter how hard sealing surfaces, e.g. diamond-coated surfaces, are used, the sealing surfaces break and wear out already after a few minutes, causing seizure of the rotating joint surface. Naturally the sealing pressure used has a considerable influence on the function of the sealing. Thus the sealing pressure should be adjustable so as to guarantee appropriate sealing in the given process conditions.
The object of the present invention is to provide a rotating nozzle structure of an extruder in which the structure of the rotating joint guarantees sufficient tightness but simultaneously allows adequately long continuous use. This is achieved with a rotating nozzle structure which is characterized in that the die is arranged to slide in the axial direction with respect to the extruder frame and comprises a flow channel which is under pressure of the material to be extruded and tapers conically towards the nozzle, and that the ratio of the area of the projection surface of the die""s annular surface to the area of the projection surface of the flow channel""s conical section is between 0.1 . . . 10. According to the invention, the sealing pressure is produced by means of the pressure of the material to be extruded, and thus the sealing pressure is proportional to this pressure. The sealing pressure is directed onto the sealing surface by providing the extrusion channel with a conically tapering shape. By dimensioning the projection surface of the conically tapering channel appropriately with respect to the area of the sealing surface, we achieve desired sealing, but avoid too high sealing pressures.
The sealing plate, which is placed between the die and the rotating nozzle and is the wearing part of the rotating joint, is preferably made of a porous material impregnated with a lubricant, such as heat-resistant oligomer or polymer with a suitable viscosity. In a preferred embodiment the sealing plate is made of a sintered bronze-based material. This solution provides a structure which will function even when the maximal rotational speed of the nozzle is 500 rounds per minute and the maximal pressure to be sealed 300 bar.
By dimensioning the sealing pressure as described above and by using a suitably lubricated sealing plate to decrease friction on the sealing surface, we achieve the object of the invention, i.e. good sealing and a sufficiently durable structure, which enables use of the rotating nozzle structure for over 24 hours without service. In practice the service means change of the sealing plate. Naturally the die is made of a hard enough material so that on the sealing surface wear is concentrated on the sealing plate, which is easy and quick to change and is a relatively inexpensive component.